A laser light source is widely used as a built-in device incorporated into industrial devices or domestic devices. A semiconductor laser diode or a solid-state laser light source is exemplified as the laser light source. A wavelength conversion laser light source is exemplified as another laser light source to obtain laser beam of a wavelength which is hardly obtained by means of direct oscillation by the semiconductor laser diode or the solid-state laser light source.
A wavelength conversion laser light source typically comprises a wavelength convertor. The wavelength conversion laser light source converts the fundamental wave laser beam (hereinafter, called “fundamental wave light”), which is incident on a wavelength convertor, into a frequency of laser beam by means of a Second Harmonic Generation element (SHG element), which generates light (called “second harmonic light” hereinafter) having twice as high as a frequency of the fundamental wave light. Alternatively, a wavelength conversion laser light source uses a Sum Frequency Generation (SFG) element, which generates light having a frequency that is a sum of two frequencies (sum frequency) of light input to a wavelength convertor, for the conversion into the frequency of the laser beam. As a result of the conversion into the frequency of the laser beam by means of a non-linear optical effect such as SHG or SFG, laser beam having a converted wavelength is output.
One of known wavelength conversion laser light source s comprises a resonator having a pair of reflecting mirrors, and a laser medium and a wavelength convertor which are situated between the paired reflective mirrors.
FIG. 16 is a schematic view of a conventional wavelength conversion laser light source. The conventional wavelength conversion laser light source is described with reference to FIG. 16.
The conventional wavelength conversion laser light source 900 has a solid laser medium 910, a wavelength convertor 920, a concave lens 930 and a dielectric multi-layer film 940. The dielectric multi-layer film 940 has high reflectivity for the fundamental wave light FL and reflectivity for the second harmonic light CL. The concave lens 930 includes a dielectric multi-layer film which has high reflectivity for the fundamental wave light FL and low reflectivity for the second harmonic light CL. Orientation of the crystals in the wavelength convertor 920, which is formed from non-linear optical crystals, and a period of the polarization reversal structure are controlled to match phases of the fundamental wave light FL and the second harmonic light CL with each other.
The concave lens 930 and the dielectric multi-layer film 940 operate as a resonator. The fundamental wave light FL inside the resonator is converted into the second harmonic light CL by the wavelength convertor 920. The second harmonic light CL, which is generated inside the resonator, is output outside the resonator via the end surface of the wavelength convertor 920.
If a cross-sectional area of the beam in the wavelength convertor of the wavelength conversion laser light source becomes small, conversion efficiency from an input power to the second harmonic wave increases. Patent Documents 1 and 2 propose forming a beam waist in the wavelength convertor to achieve efficient wavelength conversion.
According to Patent Documents 1 and 2, a reflecting mirror having a curved surface of a prescribed curvature is situated near the laser medium. A reflective coating layer which reflects the fundamental wave light with high reflectivity is formed on an end surface of the wavelength convertor. The reflecting mirror and the reflective coating layer operate as a resonator. The beam waist is formed on the end surface of the wavelength convertor. The wavelength conversion becomes efficient because a cross-sectional area of the fundamental wave light becomes small in the wavelength convertor.
The compact and inexpensive wavelength conversion laser light source disclosed in Patent Document 1 achieves high conversion efficiency. The wavelength conversion laser light source disclosed in Patent Document 2 radiates heat of the solid-state laser toward the SHG element to obtain stable output. However, these conventional technologies do not address issues about stabilization of the resonator.
Patent Document 1: JP H5-145160 A
Patent Document 2: JP 2000-183433 A